Apparatus for and method of reproducing sound



F. W. LEE

' 2 Sheets-Sheet 1 Filed May 2. 1925 FIGS . HG. z

Eff] FIG. 5

INVENTOR. FREDERICK W. LEE

BY fiat-W ATTORNEY.

March 22 1927.

APPARATUS FOR AND METHOD OF REPRODUCING souun ls FIGM 22 1927. March F. w. LEE

APPARATUS FOR AND METHOD OF REPRODUCING SOUND 2 Shoots-Sheet '2 Filed May 2. 1925 FIG.7

INVENTOR. FREDERICK W. LEE

A TTORNEY.

Patented Mar. 22, 1927.

UNITED STATES FREDERICK W. LEE, OF OWINGS MILLS, MARYLAND.

APPARATUS FOR AND METHOD 0F REPRODUCING SOUND.

- Application filed May 2, 1925. Serial m. 27,444.

This invention relates to loud speaking devices such as are used in radio ,te ephony and, more particularly, to electrostatically operated loud speaking devices, and aims further to define a method whereby to secure a distortionless sound reproduction of transmitted signals.

The object of this invention is to secure distortionless sound reproduction with the aid of a membrane in an electrostatic field.

A further object of this invention is to reduce the causes which create the disturbances in the reception of sound to a substantially irreducible minimum whereby movement of thediaphragm of the sound reproducing device is controlled entirely by the energy consuming factors.

A further object of this invention is to produce an absolutely symmetrical vibration of the diaphragm whereby to eliminate the formation of any transients during the initial impressing of a periodic force upon the diaphragm. I

A further object of this invention is the use in a sound producing instrument of the type described of a diaphragm having negligible mass and no elasticity. p

A further object is to produce a sound re.- producing instrument which is simple, inex pensive, and highly efiicient irrespective of any position in which the instrument may be positioned.

A further object of this invention is to provide an improved method for reproducing sounds in which a maximum clarity and volume of'tone is obtained with a minimum of distortion, which method involves the employment of a charged membrane in an electrostatic field.

Other objects of this invention will appear more fully hereinafter.

The invention consists substantially in the construction, combination, location and relative arrangement of parts, all as will be more fully hereinafter set forth, as, shown in the accompanying drawings, and finally pointed out in the appended claims.

In the accompanying drawings Figure 1 is a vertical sectional view of one form of this invention together with a diagrammatical circuit arrangement showing its connection to the voice frequency source; Fig. 2 is a perspective of the. elements comprising this invention; Fig. 3 is a. side elevation view thereof;

ing medium, in this instance, air.

- Fig. 4 is a view similar to Fig. 1 but showing a modification thereof;

Fig. 5 is still another modification;

. Fig. 6 is a diagram of connections showmg a modified arrangement connected to a usual receiving circuit.

Fig. 7 is a diagrammatic showing, similar to Fig. 6, but showing a modification thereof.

Figs. 8 and 9 are diagrammatic showings of Figs. 1 and 4, respectively.

The same part is designated by the same reference numeral whenever it occurs throughout the several views.

It is a well known fact that all sound vibrations of a complex nature may be resolved into simple harmonic vibrations. In other words, the sum of the simple harmonic vibrations of proper amplitude and relative phase displacement in time is equalto the resultant complex sound.- J If a diaphragm can be secured and operated in such a manner that the component simple harmonic vibrations are reproduced in the proper relative magnitudes and phase positions, then the resulting complex sound will be free from distortion. Furthermore, if a periodic force is applied to a membrane through the introduction of an electrostatic field its tend-' brane itself and its adjoinmg medium and is.

a function of three characteristic properties. First is the effect of the mass or inertia of of the membrane; second is the effect of the elasticity of the membrane; and third is the effect of the resistance of the membrane to motion due to the-reaction of the surround- The presence of mass or inertia in the membrane causes it to vibrate with a greater displacement at low frequencies than at high frequencies for a definite applied maximum harmonic force with the result that the overto' itself and it will transform energy true force with the result that the overtones are magnified. Just as in the case where mass is present in the vibrating membrane this elastic property absorbs energy as a function of the frequency and results in distortion of the sound due to the fact that the same force gives different displacements for different fre uencies.

e third characteristic, that of the re sistance offered to movement of the membrane due to energy impressed upon the air medium, is entirely independent of the frequency. In other words, the same applied force .will result in the same displacement irrespective of the frequency impressed. It is the presence of this resistance Which is offered as continuous work to be overcome. It is a well known fact that a diaphragm imparts velocity to the adjoining air when the diaphragm is vibrated and that it is this vibrating air proceeding from the diaphragm which produces the sensation of sound upon reaching the ear drum. In this sense the diaphragm may be considered as a source of sound in that it has a vibration velocity acting upon the air. The setting of the air in vibration requires a transfer of energy from the diaphragm to the air,

and consequently there .will be a reaction,

on the diaphragm tending to reduce itsv vibrational velocity. Of the three characteristic properties affecting the motional impedance of the vibrating diaphragm only two are of disturbing nature and. cause distortion of the sound. These are the mass or inertia and the elastic properties. It is among the special purposes of this invention to reduce these two isturbing causes to substantially an. irreducible minimum so that the controlling factors for imparting 'motion to the diaghragm are those which consume energy irectly. The diaphragm for this purpose must possess no mass. and no elasticity com- P pared to its resistance impedance. A diaphragm having negligible mass and elastic properties will have no natural inherent frequencies such that it would osclllate if left to the impressed'forces with the result that the sounds will be emitted entirely free'from distortion. Each harmonic will ap ear in the resultant sound wave proportiona to the electric impressed force.

That mass and elasticity are the only factors which disturb the quality of vibrational frequency, and therefore that of the sound, may be proved mathematically as follows. The force of reaction mentioned above-which tends to reduce the vibrational velocity of the diaphragm gives rise to a resistance to motion or velocity and is approximately proportional to the-extent of the motion portional to the velocity of vibration F rms, where F, is the force-"opposing motion due to the internal friction of the diaphragm, andvR, is the internal resistance to motion. I

' Diaphragms also offer impedance to motion due to their mass or inertia as set forth above. The force necessary to overcome this impedance is proportional to .the

, rate of change of velocity of the diaphragm and is expressed as follows: A

p a id;

where K is the inertia hragm. Y The resistance offered to motion due to the elasticity of the diaphragm requires a force necessary to overcome it which is pro portional to the distention'of the diaphragm, expressed as follows: F.,- S, distension.

Since distention in a. given direction is proportional'to velocity and time, the above expression may be expressed thus';

F,=S /'V dt where S is the elastic constant ofthe diahragm. i v The total force at] any instant of the dia-- phragm is the sum of the individualcom-v ponent forces:- V

constant of the dia- But the applied force F is usually, be-

These vibrations are characterized by two factors; the maximum amplitude due to each harmonic, and the phase relation of each harmonic.

The maximum amplitude of a harmonic 1s:-

. im lib S rl- 1) -nKw) .The phase relation is -n i in Kw la) rw The force F of the same amplitude but of difi'erent frequencies will not cause the same amplitudes of vibrational frequencies unless the inertia constant K and the clasticity constant S are zero so that the vibrational amplitude is a function of'the resistance only.

Thus, when K and S are each zero "'R.+R| Under this condition each harmonic is of the same relative Value and has the same relative phase displacement towards each other and the vibration of thediaphragrn is true to the impressed forces. Also under the above conditionsjhere is no transient coming into vibration after a given har- 5 monic force comes into action.

There is still another disturbin element in the true and undistorted repro notion of sound which hasas yet not been discussed. It a. given periodic impulse is impressed upon some portion of the dia hragm, for example at the center, this impu se is propagated to the edge of the diaphragm with the result that the vibrational velocities are different in diderent parts or areas of thediaphragm. This gives rise to a phenomenon similar in nature to: standing nodes and loops. The disturbingtransients encountered during the initial formation of these nodes i and loo s .are due to thetraveling wave oil and to the resistance toimparting energy to across t e diaphragm. 'It is this unsynr metrical vibration of the diaphragm which be an electrostatically charged condition. The

current ensuing from the frequency source superimposes an additional charge on that induced by the impressed unidirectional potential and tends to disturb the electrostatic balancebetween the diaphragm and its e1nbracing plates with the result that the diaphragm moves. The sole resistance tomotion which is encountered by the'diaphragrn is that due to the adjoining air. .Because the movement of the diaphragm is dependent entirely upon its attraction. or repulsion as influenced by the electrostatic field there is no need for any inherent restitutional or elastic forces to return it to normal position, and the diaphragm may vibrate in whole or in part with no elastic connections between any parts thereof. In the accompanying drawings l have shown several connections emboying my invention together with several circuit arrangements for carrying the same into practical operation. The forms shown, while W0 they are the best forms in which I contemplate at present carrying out my invention, are merely illustrative of the principles involved, as many variations in the details of structureand relative arrangement ma 1W5 occur to persons skilled in the art and sti 1 fall within the broad scope of my invention as defined in the appended'claims.

Referring to the drawings, Figures 1', 2

and 3 show one preferred form of my invention. A thin flexible diaphragm l0, 7 which is of negligible mass and elasticity,

is the vibrating element of the reproducing instrument. This diaphragm may be made of electrically conductive material or it may. 1115 a silk, paper, parchment or other such membrane coated and treated in such manner as to give it an electrically"conductive character. The. diaphragm 10 is spaced approximately symmetrically between a air of electrically conducting plates 11 an 12 and is held securely bound therebetween by means of insulating spacer rings 13 and M which extend substantially about the edge of the diaphragm The rin 13 and 14 are l? I preferably secured to the gates 11-and12, respectively, in any desire manner and in addition to acting as a supporting means for the flexible diaphragm serve also to insulate the metallic plates 11 and 12 from 'each other. The diaphragm 10 is supported between the spacer rings in a loosely stretched condition, it being only necessary to maintain its surface free from wrinkles or other irregularities. Concentrically positioned within the supporting rings 13 and 14. are mounted insulating spacer rings 15 and 16 of considerably smaller diameter. The rings 15 and 16 are likewise secured to the inner surfaces of the plates 11 and 13, respectively, but their function is merely to preclude the possibility of the diaphragm 10, while under vibratlon, from contacting with the plates. The plates 11 and 12are provided with a plurality of perforations17 throughout their entire surface and while a perforated disc or plate of this sort is to be preferred, the employment of a rigid metallic screen will give as good results.

As is diagrammatically shown in Fig. 1 a voicefrequency transformer 18 'has its primary connected to the input voice frequeney voltage source 19. The secondary terminals 20 and 21-of the transformer 18' are connected to the plates 11 and 12 at the points 22 and 23, respectively, while a tap.

eral preferred forms of this invention. 1n Fig. 4 the relative arrangement of the d1a-' phragm 10 and the plates 11 and 12 are reversed. Thus a single perforated plate is employed which is symmetrically disposed with respect to two vibrating diaphragms 31 and 32. -As in the previous case, insulating supporting washers 13 and 14 are used, as are the insulating s acer rings 15 and 16. The perforations in t e plate 30 are indidated as before by the numeral 17. The con nections leading from the transformer 18 are now reversed, that is, the terminals 20 and 21 are connected to terminals 33 and 34 of the diaphragms 31 and 32, respectively, while the connection from midpoint 24 through the battery 26 now leads to terminal 35 on the perforated plate 30. As will be described hereinafter, the principle of operation is the same.

Still another modification is that shown in Fig; 5. This shows the application of this invention to a multiple-diaphragm sound-reproducing instrument. The metallic plates are divided into two main groups and 41 each including a plurality of plates which are interleaved. Each of these plates are provided with a central opening which openings are in horizontal alinement. A.

light connecting link or rod 43 extends through the alined openings 42 and has mounted upon its freely projecting end a main or common vibrating diaphragm 44. The rod 43 carries in addition to thisiaphragm 44 a plurality of light, rigid and 70 preferably circular vanes 45 which latter are inter-spaced between the plates 40 and-41 as is clearly shown in Fig. 5. These vanes or membranes 45 together with the rod 43 and the diaphragm 44 constitute the moving 7 element of the instrument and are so suspended in space as to vibrate as a unit. In a manner similar to that already described in-connection with Fig. 1 the vibrating unit is connected through the battery 26 to the midpoint 24 of the transformer secondary, while the plates 40 and 41 forming the stationary elements are connected to the terminals 21 and 20, respectively. As in the foregoing modifications the theory of operation 8 is the same and will be described hereinafter.

Figs. 6 and 7 show still other modification"; of which my invention is capable. Each of these modifications, however, deal not with the relative arrangement of the 9 vibrating and stationary elements of the instrument, but rather with alternative arrangements for supplying the polarizing voltage to either of these elements. In Fig. 6 the dotted lines inclose a part of the usual receiving circuit, the receiver terminals of which are indicated at and 51. To these terminals is connected the primary of the voice frequency transformer 18. In this instance the transformer 18 is provided with a pair of secondaries 52 and 53, the secondary 52 having its terminals connected to the plates 11 and 12, while the secondary 53 has one terminal connected to the midpoint of the secondary 52 and its other terminal connected to the diaphragm 10 through the rectifier 54. This circuit arrangement involving the use of a rectifier eliminates the necessity for the use of a separate direct current source, the secondary 53 supplying the 1 charge to the diaphragm 10 to actuate it. Q

Fig. 7 is a modification of the arrangement in Fig. 6 and employs the ordinary transformer 18 heretofore described in connection with Figs. 1 to 5, but employs in ad- 1 dition thereto a second transformer having its primary supplied from a separate source of alternating current. The secondary terminals of transformer 60 are connected to the midpoint of the secondary of transformer 18 and to the diaphragm 10 through the rectifier 54.

In each instance above described, the diaphragm or the plate as the case-may be, is polarized by a unidirectional current voltage 1 which is connected through the transformer 18 to plates or diaphragms, respectively. Assuming it to be the diaphragm which is the polarized element, the diaphragm will carry a charge on each side thereof of like I polarity and the plates will each carryan equal but opposite charge. Upon connecting the primary of the transformer 18 to the source of voice frequency and the terminals of the secondary thereof to the plates, the charge upon each plate becomes equal to the polarizing charge plus the voice frequency charge on one side and the polarizing charge minus the voice frequency charge on the other side. Hence, the charge increases on one side and decreases on the other.

The diaphragm which was in equilibrium .before the voice frequency charge was introduced will now seek a new equilibrium position and'in so doing will move harmonically with this charge. In other words, the diaphragm will have a vibrational velocity true to the applied voltage from the transformer. Also due to the fact that the charge is spread over the entire surface of the diaphragm there is little unsymmetrical vibration of the diaphragm to cause any distortion of speech. That is, the diaphragm throughout its entire surface vibrates as a unit, each portion thereof having applied thereto a force of about equal magnitude, and in view of the fact that the diaphragm possesses no elasticity there'is a negligible elastic transmission from one portion of the diaphragm to an adjacent portion.

From the foregoing description it will be seen that I have provided a sound reproducing device whlch is dependent ,for its action upon the attraction and repulsion t a flexible disc or membrane under the inreproduced without any distortion whatever. c The shape of the diaphragm is entirely immateriaL. While in the preferred forms a dis-c shaped diaphragm is used, it may just as well be square, conical, cylindrical or any other convenient form provided parallel conducting surfaces are employed for the electric field.

While in the foregoing description the apparatus and method have been more or less described as being adapted for use in connection with the reproduction of sound, it .is to. be understood that this apparatus and, method a're equally applicable for usev in connection with the transmission of sound. This is due to the fact that the apparatus is reversible; that is, it may be used either as a receiver or as a transmitter. Accordingly, in'the appended claims, wherever the the apparatus or method be used in connection with the reception or transmission of sound.

Having now set forth the objects and nature of my invention and constructions 7 embodying the rinciples thereof, what I. claim as new an useful and desireto secure. by Letters Patent is i v 1. In a sound reproducing device of the type described, an electrically conductive vibrating element having negligible mass and elasticity characteristics, in combination With a plurality of metallic plates arranged symmetrically with respect to said vibrating element, said vibrating element being suspended freely between said plates such that any portion thereof vibrates independently of any other portion.

2. In an electrostatic sound translating instrument, in combination, a flexible membrane having the surfaces thereof treated with an electricall conductive material and having substantia ly negligible inertial and elasticity constants, a pair of electrical con ducting plates co-extensive in surface area with said flexible member and arranged sym-j metrically with respect thereto, and insulating elements disposed intermediately of said 7 I membrane and plates.

3. In an electrostatic sound translating system, a flexible membrane constituting the vibrating element thereof, means for inducing an electric charge of like polarity over its entire surface, a pair of plates spaced from each other and embracing said meml brane, means for inducing an opposite charge to the charge on said membrane won the surfaces of each of said plates, and means for superimposing an additional charge upon said plates whereby the electrostatic bal- 1.

ance between said membrane and disturbed.

4. Tn an electrostatic sound'translating s'ystem,'a fiexible'member constituting the vibrating element thereof, a pair of mem- 1 bers spaced from each other and embracing said-flexible member, means for distributing charges of electricity over the surfaces" of plates is -all of said members whereby to create an electrostatic field of, balanced forces between 1 all of said members, and means for superimposing a varying charge upon said embracing members whereby the electrostatic balance between all of said members is disturbed. 1o

5. In an electrostatic system, aplurality of symmetrically arranged electricall conductive members, means for distri uting charges of electricityover the surfaces of said members whereby to create an elecl trostaticfield of balanced forces therebetween, and additional means for superim-' posing a charge of electricity upon the other of said members whereby to unbalance said electrostatic field forces.

6. In an electrostatic system, a membrane adapted to have a charge of electricity distributed over thesurface thereof, a pair of rigid metallic plates symmetrically disposed Wltll respect to said membrane and spaced therefrom, each of the facing surfaces of said plates having an induced charge equal and opposite to said first mentioned charge distributed thereover whereby an electrostatic field is established between said membrane and said lates, and means for superimposing a furt er charge upon one of said induced charges to create an unbalancing of the forces of said electrostatic field.

7. In a sound reproducing system, in combination, a source of varying potential, a transformer having the primary thereof connected to said, potential source, a plurality of electrically conductive elements respectively connected to the secondary terminals of said transformer, a conducting member interposed between said elements, and means for placing said elements and member in an electrostatically charged condition of con stant polarity, said latter condition being influenced by said varying potential.

8. In a sound reproducing system, in combination, a source of varying potential, a transformer having the primary thereof connected to said potential source a plurality of electrically conductive elements respectivelyconnected to the secondary terminals of-said transformer, a conducting member symmetrically disposed between and insulated from said elements, and a polarizing voltage source interposed between said latter member and the midpoint of said transformer secondary.

'9. In a sound translating system, in combination, means for transferring ener y of varying potential, a plurality of electrically conductive members secured together but insulated from each other, certain of said members being connected to the terminals of said means, an electrically conductive diaphragm disposed between said members, and

a unidirectional current. source connected to said diaphragm and to the'midpoint of and then causing an unbalancing of the field forces wherebysaid vibrating element is caused to move in said field as it seeks to re establish the balance of said forces and' thereby produce sound.

12. The method for propagating sound substantially free from distortion which con sists in connecting a pair of electrically conductive members to a source of varying potential, then connecting a third conductive member to a source of polarizing voltage to create a balanced electrostatic field between all of said members, and finally disturbing the balance of the field forces to cause said third member to move as the potential varies.

13. The mehod for translating speech and other sounds substantially free fi'om distortion which consists in distributing a charge of electricity over the entire surface of an electrically conductive member, then inducing an opposite charge over the embracing surfaces of a pair of'conductive members, and finally superimposing an additional charge on one of said latter surfaces whereby said first mentioned member is caused to move to an adjusted position.

14. The method for propagating sound subtantially free from distortion which consists in placing a uniformly distributed charge over the surface of the vibrating element of the sound reproducing instrument, then impressing a periodic potential upon electrically conductive elements spaced on each side of said vibrating element whereby the charges on each of said elements induced by said first mentioned charge is increased in one case and decreased in the other to cause said vibrating element to move with' signature.

, FREDERICK WL LEE. 

